1. Field of the Invention
This invention relates in general to switching regulators and more specifically to a method of controlling the charging of a bootstrap capacitance which is incorporated into a switching regulator of a power regulator connected to an electric load.
2. Discussion of the Related Art
As is well known, many applications in the electric industry require that the value of a current through an electric load be regulated.
The most commonly adopted solution for regulating a lower output voltage than the input voltage is to use a switching regulator of the step-down type. In this case, the current through the electric load is regulated by means of a power transistor which is controlled from a driver circuit.
The state of the art favors the use of MOS transistors as the power switches, in preference to bipolar transistors. The provision of a MOS transistor affords improved efficiency for the regulator as a whole; it also involves, however, added circuit complexity in that a second power supply, higher than that to be applied to the drain terminal, must be provided for charging the gate terminal of the MOS transistor.
Several prior solutions are available for producing the aforementioned second power supply, of which the most commonly adopted one provides for the use of a bootstrap capacitance which can be re-charged during the conduction phase of a recirculation diode. Other, and more complex, solutions, such as the provision of a step-up circuit for producing the desired power supply, involve an increased number of outward connections for the integrated circuit. It has also been proposed to use an internal charge pump, but this solution cannot provide the amount of charge required for fast changeovers of the MOS switch.
In the respect of the first-mentioned solution, the use of a bootstrap capacitance restricts the operational conditions of the switching regulator. In fact, where the voltage value to be regulated exceeds the difference between the voltage value to which the bootstrap capacitance is charged and the turn-on threshold of the MOS switch, the regulating system can only operate properly if the load output current is larger than a minimum current I.sub.MIN.
To illustrate this concept, a review of the operation of a switching regulator 2 of the step-down type may be helpful. The bootstrap capacitance is powered from a voltage generator V.sub.REG 3 having a diode D2 9 in a series therewith, as shown in the accompanying FIG. 1.
A MOS transistor M1 8 operates as a switch to regulate the current being supplied to an electric load LOAD 5. For the purpose, the switch M1 has a first conduction terminal connected to a supply voltage reference Vcc, and a second conduction terminal OUT connected to the load LOAD through an inductance L 1. A diode D1 10 is connected between the terminal OUT and one end of the LOAD 5 taken to a ground GND. A capacitor C1 4 is provided in parallel with the LOAD 5. The gate terminal of the switch M1 is connected to the output of a driver circuit DRIVER 7.
With the switch M1 in the off state, the current to the inductance L 1 flows through the diode D1 10, presently conducting, so that the voltage at the node OUT will turn negative and be equal to -V.sub.D1. Under this condition, the voltage generator V.sub.REG is able to deliver a current for charging the bootstrap capacitance C.sub.BOOT 6. The maximum voltage C.sub.BOOT 6 at that capacitance is given by: EQU C.sub.BOOTMAX =V.sub.REG -V.sub.D2 -(-V.sub.D1).apprxeq.V.sub.REG ;
With D1 conducting, V.sub.REG will deliver a current until V.sub.cboot becomes less than C.sub.BOOTMAX, In operation at a small load current, there is a time period T1 when the current IL at the inductance L becomes zero, as shown in FIG. 2C. In this case, at the end of the discharge transient, the voltage V.sub.OUT at the node OUT becomes equal to Vload, as shown in FIG. 2B.
Referring now to FIGS. 3A-3E, it is shown that the bootstrap capacitance can only be charged during the time when the recirculation diode D1 is conducting, as shown in FIG. 3D. If the average current demanded by the load is a very small one, the pulses SWITCH for turning on the switch M1 are quite narrow and have a very large period, as shown in FIG. 3A, because a small current will suffice to regulate the output voltage Vload. At the end of the turn-on pulse, following a short time period of conduction of the diode D1 when the bootstrap capacitance C.sub.BOOT is being charged by the generator V.sub.REG, the inductance current IL drops to zero, and the voltage V.sub.OUT at the node OUT becomes equal to Vload. Under this condition, the static consumption driver of the Idriver stage results in the bootstrap capacitance being gradually discharged. This discharge continues until the voltage V.sub.CBOOT across the capacitance equals the difference between V.sub.REG -V.sub.D2 and Vload, as shown in FIG. 3D.
Under these conditions, in order for the switch M1 to change over at the next turn-on pulse, the voltage at the bootstrap capacitance should be higher than the turn-on threshold V.sub.TH of the NMOS transistor M1, i.e. : EQU V.sub.REG -V.sub.D2 -V.sub.LOAD .gtoreq.V.sub.TH ;
Given that V.sub.MAX =V.sub.REG -V.sub.D2 -V.sub.TH ; if the voltage to be regulated is higher than V.sub.MAX, then the switching regulator will only operate properly at larger currents than a minimum value I.sub.MIN which is proportional to the consumption of the driver circuit. With currents below a value I.sub.MIN, the output voltage Vload will equal V.sub.MAX.
In actual constructions of step-down switching regulators, the critical current for proper operation of the circuit is much larger than the theoretical value of I.sub.MIN, because the considerations made above takes no account of the less-than-ideal nature of the voltage generator V.sub.REG. In fact, no real generator would be able to deliver its maximum current at once, especially when constructed for a small drop, as is usual in most instances. By way of example, FIGS. 4A and 4B shows the current I(V.sub.REG) to be delivered by the generator V.sub.REG upon the diode D1 being turned on.
In a condition of minimum load, the switch M1 would be held "on" for a very short time, and the amount of charge fed to the bootstrap capacitance from V.sub.REG would be less than optimum, as shown in FIGS. 5A and 5B, where the triangular areas in FIG. 5B, represent the amounts of charge.
The underlying technical problem of this invention is to provide a method for optimising the charging of a bootstrap capacitance during operation of a switching circuit of the step-down type, which method can obviate the drawbacks with which prior switching regulators have been beset.